In the polymerization, main aim is the achievement of desired quality product. For this reason, control and design is very important at polymerization processes. In this study, polymerization of styrene in a jacketed semi-batch reactor was examined. Manipulated variable was the heat Q which was given from electrical heater. Fuzzy control was chosen as the control method in this study. Membership functions and relation matrix which provide the best fuzzy control were obtained with GA. The fitness function for GA was chosen as the integral of the absolute value of error, IAE. In experimental studies, styrene and toluene mixture was fed continuously to the reactor. As the initiator, benzoyl peroxide was used. Fuzzy control method was used to keep the desired temperature set point. Because reactor temperature is increasing as a result of exothermic reaction. During the experiments, the concentration and molecular weight of polymer were measured analytically by taking samples. It is concluded that GA with fuzzy control is a good way to control the temperature of semi-batch styrene polymerization reactor. It was seen that experimental monomer conversion values are higher than the theoretical values, and viscosity average molecular weights which were obtained experimentally are acceptable when compared with numerical average molecular weights.